ISDN Physical Layer - Advanced Communication Network - Lecture Notes, Study notes of Data Communication Systems and Computer Networks

Download ISDN Physical Layer - Advanced Communication Network - Lecture Notes and more Study notes Data Communication Systems and Computer Networks in PDF only on Docsity! ISDN Physical Layer 2 4.1 Basic User-Network Interface • Physical layer functions (at reference point S or T) – encoding of digital data for transmission across the interface – full-duplex transmission of B channel data – full-duplex of transmission of D channel data – multiplexing of channels to form basic or primary access – activation and deactivation of physical circuit – power feeding from network termination to the terminal – terminal identification – faulty terminal isolation – D channel contention access (for multipoint configuration in basic) • Layer 1 specification is defined in I.430 • Key aspects of basic interface (supports a 2B+D at 192 kbps) – Line coding – Physical connector – Framing and multiplexing – Contention resolution for multidrop configurations Line Coding • full-duplex digital data transmission (one line for each direction) • no echo cancellation or time-compression multiplexing needed • use of pseudoternary coding scheme – “1”: absence of voltage – “0”: a positive or negative pulse of 750 mV ± 10% – data rate is 192 kbps Docsity.com ISDN Physical Layer 3 Basic Access Physical Connector • Physical connection between a TE and an NT: ISO 8887 • eight-pin ISDN physical connector • for twisted-pair connections • power transfer from the network Reference configuration for signal transmission and power feeding using only six leads out of eight Docsity.com ISDN Physical Layer 6 • Three types of traffic considered in contention resolution – B channel traffic-each channel is dedicated to one TE, no additional functionality is thus needed – Incoming D channel traffic-available for all devices, thus contention exists. Used LAPD addressing scheme to solve – Outgoing D channel traffic-only one device to transmit at a time • Contention resolution algorithm on D channel – In the absence of LAPD frames, subscriber transmits a series of “1” on the D channel (pseudoternary encoding → no line signal) – NT reflects back the binary value as D ch echo bit (E bit) – When a terminal is ready to transmit an LAPD frame, it listens to the incoming D ch echo bits. If it detects a string of 1-bits of length equal to threshold Xi, it may transmit, otherwise wait. • Priority mechanism based on the threshold value Xi – signaling information is given priority over packet information – a station begins at normal priority, then reduced to lower priority • Signaling information – Normal Priority X1= 8, Lower Priority X1=9 • Nonsignaling information – Normal Priority X2=10, Lower Priority X2=11 Contention resolution Docsity.com ISDN Physical Layer 7 • Three TEs are attempting to use the D channel. • Each TE maintains two priority values, X1 and X2, corresponding to signaling and nonsignaling information to be transmitted on the D channel. Each of these values is initialized to a normal priority level. When a TE has D channel information of class i to transmit, it waits until it sees a string of 1 bits on the E channel equal to Xi and then transmits. This causes the corresponding priority value to be placed at lower priority. • To recover to normal priority, a TE listens for consecutive E bits equal to 1. When the TE observes a string of 1 bits on the E channel equal to the value of the lower level of priority, it changes the priority for that class back to the value for the normal level of priority. Example of contention resolution Docsity.com ISDN Physical Layer 8 4.2 Primary Rate User-Network Interface • Like basic interface to multiplex multiple channels on a single medium • Only point-to-point configuration is allowed • Typical interface: at T reference point with a digital PBX • Two data rates defined: 1.544 Mbps and 2.048 Mbps Interface at 1.544 Mbps • based on N. America DS-1 transmission structure used on T1 • bit stream of repetitive 193-bit frame (24X8-bit time slots + a framing bit) • frames repeated at a rate of one every 125µs (8000 frames/sec) Thus each channel supports 64-kbps data rate • supporting 23 B + 1 D channels (or 24 B channels and comb. of H ch’s) FAS: frame-alignment signal O&M: operations and maintenance CRC: cyclic redundancy check Assignment of 24 framing bits across the 24-frame multiframe -Synchronization by FAS: 001011 -6-bit CRC composed by ei bits -line coding: AMI using B8ZS Docsity.com